Directories and files are managed in a file system in a UNIX(trademark)-based OS (Operating System) by the use of file management information which is referred to as meta-data. The meta-data includes an inode, an extended attribute, and the like. A disk-based file system such as UFS (Unix File System) or ext3 (third extended file system) is known as such a file system.
An inode set is associated with a file or a directory and stores information regarding the file or the directory. In the case of a file, for example, file size, the number of a block in which data is stored, a time stamp, an access right, and the like are stored in an inode. In the case of a directory, the number of a block in which a directory entry is stored is stored in an inode. The directory entry is data for managing the name and inode number of each file in the directory. The directory entry is also included in the meta-data.
The size of an inode is fixed. Accordingly, additional information which cannot be stored in an inode is stored as an extended attribute in a disk area other than an inode. An extended attribute is information which is a combination of an attribute name and a value. For example, an ACL (Access Control List) used for managing a file access right is information which is added as an extended attribute. If an extended attribute is set, the number of a block in which the extended attribute is stored is stored in an inode.
By the way, a large-scale computer system includes a vast number of files and file access occurs frequently. Each time data in a file is accessed by a file system, access to meta-data and access to the actual data in the file occur. Accordingly, a technique for increasing the efficiency of file access by storing and managing the meta-data and the actual data in the file in different disk units and managing them is proposed. By storing the meta-data and the actual data in different disk units, the frequency of access to one disk unit can be decreased and file access speed in the entire system can be improved.
Japanese Laid-open Patent Publication No. 2003-280950
Japanese Laid-open Patent Publication No. 2007-200029
With conventional file systems, however, the size of an area in which an extended attribute is stored is fixed. This is the same with an inode. As a result, the extensibility of a file management function is limited. In the case of a large-scale system, for example, a vast number of users use the system. In this case, information indicative of a file access right for each user is set as an extended attribute. However, the size of an area which stores an extended attribute is fixed, so an amount by which information indicative of an access right can be stored has an upper limit. As a result, the number of users for which access rights can be set also has an upper limit. In the past, limits were set in this way on the extensibility of a file management function and proper file management suitable for a large-scale system could not be performed.